In general, it is known that a polysaccharide (of a hydrophilic nature) appropriately functionalized with molecules of a hydrophobic nature can provide an assembling system with nanohydrogel characteristics if exposed to particular conditions in an aqueous environment.
Nanohydrogels are acquiring a certain importance in the pharmaceutical field since, if they are rendered sterile and apyrogenic, they can be used as compounds vehicling the drugs and be administered both in humans and in animals via inhalatory or parenteral route (i.v., i.m., s.c.,) or else topically, with the aid of an appropriate device.
There are currently known various methods for preparing nanohydrogels starting from functionalized polysaccharides.
A first of these methods consists in subjecting the functionalized polysaccharide to sonication. Ultrasonic vibrations are able to induce formation of nanohydrogels of small dimensions. Ultrasounds generate in the polymeric suspension micro-bubbles that by imploding give rise to the phenomenon of cavitation, which promotes separation of the polymeric chains, thus favouring formation of a nanoparticle suspension. This technique, however, presents numerous disadvantages at an industrial level such as high polydispersion of the specimen, high costs, and an enormous production of heat.
Another method consists in solubilizing the functionalized polysaccharide in an appropriate solvent and adding drop by drop the solution obtained in water. In these conditions, the system precipitates, inducing formation of nanoparticles. This method of preparation presents disadvantages as regards the particularly high costs, complex manual operations that are hard to reproduce, and very long preparation times.
Furthermore, this methodology envisages the use of organic solvents, with obvious disadvantages in terms of toxicity and safety that these represent.
Yet another method consists in subjecting to dialysis against water or aqueous solution the functionalized polysaccharide once this has been solubilized in an organic solvent. The slow entry of water through the dialysis tubes causes formation of nanohydrogels of small dimensions by spontaneous self-assembly. The disadvantages that this method involves regard the presence of aggregates in a more or less significant amount, lack of reproducibility, particularly high costs, and particularly long preparation times. Furthermore, also in this case the presence of organic solvents raises problems of safety and toxicity.
As mentioned above, one of the possible applications of nanohydrogels is the one regarding pharmaceutical preparations administered via parenteral route. Nanohydrogels, in fact, can englobe a pharmacologically active principle and function as carrier for its administration.
In this context, a treatment of sterilization of the nanohydrogels becomes indispensable. The methods of sterilization used by pharmaceutical industries are not, however, totally satisfactory.
One of the main sterilization methods used is filtration by means of filters with a porosity equal to or less than 0.22 μm, following the pharmacopoeia recommendations. Even though filtration is possible, as a rule, with systems of suitable dimensions, it is in any case frequently problematical on account of clogging of the filters themselves due to the interactions that may arise between the nanoparticles and the materials constituting the filters. Furthermore, it has been found that filtration may cause, as a mechanical effect, destructuring of the nanoparticles, for example vesicles such as liposomes, causing loss from the medicament of the bio-active molecules, which remain trapped on the filter, and/or their leakage into the transport liquids.
Another sterilization method consists in irradiation with gamma rays or with a electron flow. This procedure presents the disadvantage of being able to alter the structure of the fragile bio-active molecules, cause a degradation of the polymers that constitute the pharmaceutical form, and alter the integrity of the phospholipids constituting the liposomes.
Another method used for sterilization moreover envisages the use of gases, such as ethylene oxide; this technique, however, is not easy to implement in the presence of substances that can react with the gas itself. Furthermore, also the intimate contact with the pharmaceutical forms, which is necessary to achieve sterility, may be problematical, as likewise removal of the gas prior to packaging of the pharmaceutical form itself.
There is hence felt the need to provide a methodology that will be able to prepare nanohydrogels and to sterilize them without incurring in the drawbacks of the known art.
An extremely simple and economically advantageous method for preparing directly sterile nanohydrogels with a high dimensional homogeneity has been unexpectedly found by the inventors of the present patent application.